(1) Field of the Invention
The present invention relates to a polyester vessel and a package comprising the same. More particularly, the present invention relates to a polyethylene terephthalate vessel which has a good flavor-retaining property to a content and is excellent in the impact strength of the bottom portion, the resistance to environmental stress cracking and the resistance to the shrinkage with the lapse of time (post shrinkage), and a packaging comprising the same.
(2) Description of the Related Art
A biaxially draw-blow-formed polyethylene terephthalate vessel is excellent in pressure resistance, rigidity, transparency and impact resistance and has a relatively low permeability to gases such as oxygen and carbon dioxide gas, and therefore, this vessel is widely used for containing various liquid foods, seasonings, cosmetics and the like. Moreover, a plastic can formed by wrap-seaming an opening of a polyethylene terephthalate vessel barrel with a metal lid is now being put into practical use.
However, at the step of heat forming of polyethylene terephthalate, acetaldehyde is generated by thermal decomposition, and acetaldehyde in the vessel wall migrates into the content to degrade the flavor of the content. This reduction of the flavor is especially conspicuous in a package formed by filling a content containing an edible oil in a vessel and sealing the vessel.
As the conventional means for reducing the concentration of acetaldehyde formed by the thermal decomposition in polyethylene terephthalate, there can be mentioned a method in which after ordinary polymerization, polyethylene terephthalate is further subjected to solid phase polymerization to decrease the contents of low-molecular-weigth components or increase the molecular weight of polyethylene terephthalate to some extend, a method in which the form temperature at the heat-forming step is reduced to a level as low as possible, and a method in which the shear stress at the heat-forming step is reduced to a level as low as possible.
In general, a vessel of a polyester such as polyethylene terephthalate is prepared by melting the polyester and injecting the melt into an injection mold or by once forming a substantially amorphous preform by the above-mentioned injection molding and draw-forming the preform at a draw-forming temperature. Increase of the molecular weight of the polyester tends to result in degradation of the injection moldability of the polyester, and reduction of the molding temperature results in increase of the load imposed on an injection machine. Moreover, reduction of the shear stress results in reduction of the molding speed and tends to cause inclusion of air bubbles.